Programmable structure, an array including the structure, and methods of forming the same

ABSTRACT

A microelectronic programmable structure suitable for storing information, and array including the structure and methods of forming and programming the structure are disclosed. The programmable structure generally includes an ion conductor and a plurality of electrodes. Electrical properties of the structure may be altered by applying energy to the structure, and thus information may be stored using the structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/390,268 entitled PROGRAMMABLE STRUCTURE, AN ARRAY INCLUDINGTHE STRUCTURE, AND METHODS OF FORMING THE SAME, filed Mar. 17, 2003(“the '268 Application”), which is a continuation-in-part of U.S. patentapplication Ser. No. 10/268,107, entitled PROGRAMMABLE MICROELECTRONICDEVICE, STRUCTURE, AND SYSTEM AND METHOD OF FORMING THE SAME, filed Oct.9, 2002, which is a continuation-in-part of U.S. patent application Ser.No. 10/118,276 entitled MICROELECTRONIC DEVICE, STRUCTURE, AND SYSTEM,INCLUDING A MEMORY STRUCTURE HAVING A VARIABLE PROGRAMMABLE PROPERTY ANDMETHOD OF FORMING SAME, filed Apr. 9, 2002, which is acontinuation-in-part of U.S. patent application Ser. No. 09/502,915,entitled PROGRAMMABLE MICROELECTRONIC DEVICES AND METHODS OF FORMING ANDPROGRAMMING SAME, filed Apr. 19, 2000; the '268 Applicaiton is also acontinuation-in-part of U.S. patent application Ser. No. 09/951,882,entitled MICROELECTRONIC PROGRAMMABLE DEVICE AND METHODS OF FORMING ANDPROGRAMMING THE SAME, filed Sep. 10, 2001; and the '268 Applicationclaims the benefit of U.S. Provisional Application Ser. No. 60/364,547,entitled PASSIVE MEMORY ARRAYS USING THE PROGRAMMABLE METALLIZATIONCELL, filed Mar. 15, 2002, U.S. Provisional Application Ser. No.60/365,551, entitled PULSE MODE PROGRAMMABLE METALLIZATION CELL (PMPMC)TECHNOLOGY, filed Mar. 18, 2002, U.S. Provisional Application Ser. No.60/365,602, entitled METHOD AND APPARATUS TO INCREASE SPEED OFELECTRODEPOSIT FORMATION AND ELECTRODEPOSIT DENSITY IN PROGRAMMABLEMETALLIZATION CELL DEVICES, filed Mar. 18, 2002, and U.S. ProvisionalApplication Ser. No. 60/365,601, entitled PROGRAMMABLE METALLIZATIONCELL MATERIALS, STRUCTURES AND DEVICES, filed Mar. 18, 2002.

FIELD OF INVENTION

The present invention generally relates to programmable microelectronicdevices. More particularly, the invention relates to programmablemicroelectronic structures and devices having an electrical propertythat can be variably programmed by manipulating an amount of energysupplied to the structure during a programming function and to memoryarrays including the structures.

BACKGROUND OF THE INVENTION

Memory devices are often used in electronic systems and computers tostore information in the form of binary data. These memory devices maybe characterized into various types, each type having associated with itvarious advantages and disadvantages.

For example, random access memory (“RAM”), which may be found inpersonal computers, is typically volatile semiconductor memory; in otherwords, the stored data is lost if the power source is disconnected orremoved. Dynamic RAM (“DRAM”) is particularly volatile in that it mustbe “refreshed” (i.e., recharged) every few hundred milliseconds in orderto maintain the stored data. Static RAM (“SRAM”) will hold the dataafter one writing so long as the power source is maintained; once thepower source is disconnected, however, the data is lost. Thus, in thesevolatile memory configurations, information is only retained so long asthe power to the system is not turned off. In general, these RAM devicescan take up significant chip area and therefore may be expensive tomanufacture and consume relatively large amounts of energy for datastorage. Accordingly, improved memory devices suitable for use inpersonal computers and the like are desirable.

Other storage devices such as magnetic storage devices (e.g., floppydisks, hard disks and magnetic tape) as well as other systems, such asoptical disks, CD-RW and DVD-RW are non-volatile, have extremely highcapacity, and can be rewritten many times. Unfortunately, these memorydevices are physically large, are shock/vibration-sensitive, requireexpensive mechanical drives, and may consume relatively large amounts ofpower. These negative aspects make such memory devices non-ideal for lowpower portable applications such as lap-top and palm-top computers,personal digital assistants (“PDAs”), and the like.

Due, at least in part, to a rapidly growing numbers of compact,low-power portable computer systems and hand-held appliances in whichstored information changes regularly, low energy read/writesemiconductor memories have become increasingly desirable andwidespread. Furthermore, because these portable systems often requiredata storage when the power is turned off, non-volatile storage deviceare desired for use in such systems.

One type of programmable semiconductor non-volatile memory devicesuitable for use in such systems is a programmable read-only memory(“PROM”) device. One type of PROM, a write-once read-many (“WORM”)device, uses an array of fusible links. Once programmed, the WORM devicecannot be reprogrammed.

Other forms of PROM devices include erasable PROM (“EPROM”) andelectrically erasable PROM (EEPROM) devices, which are alterable afteran initial programming. EPROM devices generally require an erase stepinvolving exposure to ultra violet light prior to programming thedevice. Thus, such devices are generally not well suited for use inportable electronic devices. EEPROM devices are generally easier toprogram, but suffer from other deficiencies. In particular, EEPROMdevices are relatively complex, are relatively difficult to manufacture,and are relatively large. Furthermore, a circuit including EEPROMdevices must withstand the high voltages necessary to program thedevice. Consequently, EEPROM cost per bit of memory capacity isextremely high compared with other means of data storage. Anotherdisadvantage of EEPROM devices is that, although they can retain datawithout having the power source connected, they require relatively largeamounts of power to program. This power drain can be considerable in acompact portable system powered by a battery.

Various hand-held appliances such as PDAs, portable phones, and the likeas well as other electronic systems generally include a memory devicecoupled to a microprocessor and/or microcontroller formed on a separatesubstrate. For example, portable computing systems include amicroprocessor and one or more memory chips coupled to a printed circuitboard.

Forming memory devices and the microprocessor on separate substrates maybe undesirable for several reasons. For example, forming various typesof memory on separate substrate may be relatively expensive, may requirerelatively long transmission paths to communicate between the memorydevices and any associated electronic device, and may require arelatively large amount of room within a system. Accordingly, memorystructures that may be formed on the same substrate as anotherelectronic device and methods of forming the same are desired.Furthermore, this memory technology desirably operates at a relativelylow voltage while providing high speed memory with high storage densityand a low manufacturing cost.

SUMMARY OF THE INVENTION

The present invention provides improved microelectronic programmabledevices, structures, and systems and methods of forming the same. Moreparticularly, the invention provides programmable structures that can bevariably programmed depending on an amount of energy used to program thedevice. Such structures can replace both traditional nonvolatile andvolatile forms of memory and can be formed on the same substrate asand/or overlying another microelectronic device.

The ways in which the present invention addresses various drawbacks ofnow-known programmable devices are discussed in greater detail below.However, in general, the present invention provides a programmabledevice that is relatively easy and inexpensive to manufacture, which isrelatively easy to program, and which can be variably programmed.

In accordance with one exemplary embodiment of the present invention, aprogrammable structure includes an ion conductor and at least twoelectrodes. The structure is configured such that when a bias is appliedacross two electrodes, one or more electrical properties of thestructure change. In accordance with one aspect of this embodiment, aresistance across the structure changes when a bias is applied acrossthe electrodes. In accordance with other aspects of this embodiment, acapacitance or other electrical property of the structure changes uponapplication of a bias across the electrodes. In accordance with afurther aspect of this embodiment, an amount of change in theprogrammable property is manipulated by altering (e.g., thermally orelectrically) an amount of energy used to program the device. One ormore of these electrical changes and/or the amount of change maysuitably be detected. Thus, stored information may be retrieved from acircuit including the structure.

In accordance with another exemplary embodiment of the invention, aprogrammable structure includes an ion conductor, at least twoelectrodes, and a barrier interposed between at least a portion of oneof the electrodes and the ion conductor. In accordance with one aspectof this embodiment, the barrier material includes a material configuredto reduce diffusion of ions between the ion conductor and at least oneelectrode. In accordance with another aspect, the barrier materialincludes an insulating or high-resistance material. In accordance withyet another aspect of this embodiment, the barrier includes materialthat conducts ions, but which is relatively resistant to the conductionof electrons.

In accordance with another exemplary embodiment of the invention, aprogrammable microelectronic structure is formed on a surface of asubstrate by forming a first electrode on the substrate, depositing alayer of ion conductor material over the first electrode, and depositingconductive material onto the ion conductor material. In accordance withone aspect of this embodiment, a solid solution including the ionconductor and excess conductive material is formed by dissolving (e.g.,via thermal and/or photodissolution) a portion of the conductivematerial in the ion conductor. In accordance with a further aspect, onlya portion of the conductive material is dissolved, such that a portionof the conductive material remains on a surface of the ion conductor toform an electrode on a surface of the ion conductor material. Inaccordance with another aspect of this embodiment of the invention, astructure including a high-resistance region is formed by dissolving aportion of the electrode such that a portion of the ion conductorincludes a high concentration of the electrode material and anotherportion of the ion conductor includes a low concentration of theelectrode material, such that the portion of the ion conductor with alow concentration of the electrode material forms a high resistanceregion within the structure.

In accordance with another embodiment of the invention, a lateralprogrammable structure is formed by forming an ion conductor layeroverlying a substrate and forming two or more electrode structures incontact with the ion conductor layer. In accordance with one aspect ofthis embodiment, the structure includes a conductive layer adjacent theion conductor to facilitate rapid growth of a conductive region withinor on the ion conductor.

In accordance with another embodiment of the invention, a programmabledevice may be formed on a surface of a substrate. In accordance with oneaspect of this embodiment, the substrate includes a microelectroniccircuit. In accordance with a further aspect of this embodiment, thememory device is formed overlying the microelectronic circuit andconductive lines between the microelectronic circuit and the memory areformed using conductive wiring schemes within the substrate and thememory device. This configuration allows transmission of more bits ofinformation per bus line.

In accordance with a further exemplary embodiment of the invention,multiple bits of information are stored in a single programmablestructure. In accordance with one aspect of this embodiment, aprogrammable structure includes a floating electrode interposed betweentwo additional electrodes.

In accordance with yet another embodiment of the invention, multipleprogrammable devices are coupled together using a common electrode(e.g., a common anode or a common cathode).

In accordance with yet a further exemplary embodiment of the presentinvention, a capacitance of a programmable structure is altered bycausing ions within an ion conductor of the structure to migrate.

In accordance with yet another embodiment of the invention, a volatilityof a memory cell in accordance with the present invention is manipulatedby altering an amount of energy used during a write process for thememory. In accordance with this embodiment of the invention, higherenergy is used to form nonvolatile memory, while lower energy is used toform volatile memory. Thus, a single memory device, formed on a singlesubstrate, may include both nonvolatile and volatile portions. Inaccordance with a further aspect of this embodiment, the relativevolatility of one or more portions of the memory may be altered at anytime by changing an amount of energy supplied to a portion of the memoryduring a write process.

In accordance with additional embodiment of the invention, thestructures include an additional conductive layer proximate one or moreof the electrodes to improve the speed of the read/write process.

In accordance with yet another embodiment of the invention, pulse modeprogramming is used to read and write information. In this case,information can be retrieved from the device using a destructive read ora destructive write process.

In accordance with yet another embodiment of the invention, aprogrammable structure includes an additional electrode for sensing astate (0 or 1) of the programmable device.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims, considered inconnection with the figures, wherein like reference numbers refer tosimilar elements throughout the figures, and:

FIGS. 1 and 2 are cross-sectional illustrations of a programmablestructure formed on a surface of a substrate in accordance with thepresent invention;

FIGS. 3-5 cross-sectional illustration of a programmable structure inaccordance with another embodiment of the present invention.Illustrating phase-separated ion conductors;

FIGS. 6, 8, and 9 are current-voltage diagrams illustrating current andvoltage characteristics of the devices of the present invention;

FIGS. 7, 18, 19, and 25-27 illustrate arrays of programmable structuresin accordance with the present invention;

FIGS. 10-14 illustrate programmable structures including barrier layersin accordance with exemplary embodiments of the invention;

FIG. 15 is a cross-sectional illustration of a programmable structure inaccordance with yet another embodiment of the present invention;

FIGS. 16 and 17 are schematic illustrations of a portion of a memorydevice in accordance with an exemplary embodiment of the presentinvention;

FIGS. 20-24 are illustrations of programmable structures having anadditional electrode for sensing a state of the structures;

FIGS. 28-30 are illustrations of programmable structures in accordancewith additional embodiments of the invention; and

FIG. 31 illustrates tunable resonator formed using a structure inaccordance with the present invention.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated relative to other elements to help toimprove understanding of embodiments of the present invention.

DETAILED DESCRIPTION

The present invention generally relates to programmable microelectronicdevices, to arrays including the devices, and to methods of forming thedevices and arrays.

FIGS. 1 and 2 illustrate programmable microelectronic structures 100 and200 formed on a surface of a substrate 110 in accordance with anexemplary embodiment of the present invention. Structures 100 and 200include electrodes 120 and 130, an ion conductor 140, and optionallyinclude buffer or barrier layers or regions 155 and/or 255.

Generally, structures 100 and 200 are configured such that when a biasgreater than a threshold voltage (V_(T)), discussed in more detailbelow, is applied across electrodes 120 and 130, the electricalproperties of structure 100 change. For example, in accordance with oneembodiment of the invention, as a voltage V∃V_(T) is applied acrosselectrodes 120 and 130, conductive ions within ion conductor 140 beginto migrate and form a region 160 having an increased conductivitycompared to the bulk ion conductor (e.g., an electrodeposit) at or nearthe more negative of electrodes 120 and 130. As region 160 forms, theresistance between electrodes 120 and 130 decreases, and otherelectrical properties may also change.

In the absence of any barriers, which are discussed in more detailbelow, the threshold voltage required to grow region 160 from oneelectrode toward the other and thereby significantly reduce theresistance of the device is approximately a few hundred millivolts. Ifthe same voltage is applied in reverse, region 160 will dissolve backinto the ion conductor and the device will return to a high resistancestate. In a similar fashion, an effective barrier height of a diode thatforms between an ion conductor and an electrode can be reduced bygrowing region 160; thus current flow may be increased through thestructure, even in the resistance of the structure is substantially thesame.

Structures 100 and 200 may be used to store information and thus may beused in memory circuits. For example, structure 100 or otherprogrammable structures in accordance with the present invention maysuitably be used in memory devices to replace DRAM, SRAM, PROM, EPROM,EEPROM devices, or any combination of such memory. In addition,programmable structures of the present invention may be used for otherapplications where programming or changing of electrical properties of aportion of an electrical circuit are desired.

In accordance with various embodiments of the invention, the volatilityof programmable memory (e.g., cell 100 or 200) can be manipulated byaltering an amount of energy (e.g., altering time, current, voltage,thermal energy, and/or the like) applied during a write process. In thecase where region 160 forms during a write process, the greater theamount of energy (having a voltage greater than the threshold voltagefor the write process) applied during the write process, the greater thegrowth of region 160 and hence the less volatile the memory. Conversely,relatively volatile, easily erased memory can be formed by supplyingrelatively little energy to the cell. Thus, relatively volatile memorycan be formed using the same or similar structures used to formnonvolatile memory, and less energy can be used to form thevolatile/easily erased memory. Use of less energy is particularlydesirable in portable electronic devices that depend on stored energyfor operation. The volatile and nonvolatile memory may be formed on thesame substrate and partitioned or separated from each other such thateach partition is dedicated to either volatile or nonvolatile memory;or, an array of memory cells may be configured as volatile ornonvolatile memory using programming techniques, such that theconfiguration (i.e., volatile or nonvolatile) of the memory can bealtered by changing an amount of energy supplied during programming therespective portions of the memory array.

Referring again to FIGS. 1 and 2, substrate 110 may include any suitablematerial. For example, substrate 110 may include semiconductive,conductive, semiinsulative, insulative material, or any combination ofsuch materials. In accordance with one embodiment of the invention,substrate 110 includes an insulating material 112 and a portion 114including a microelectronic devices formed using a portion of thesubstrate. Layer 112 and portion 114 may be separated by additionallayers (not shown) such as, for example, layers typically used to formintegrated circuits. Because the programmable structures can be formedover insulating or other materials, the programmable structures of thepresent invention are particularly well suited for applications wheresubstrate (e.g., semiconductor material) space is a premium. Inaddition, forming a memory cell overlying a microelectronic device maybe advantageous because such a configuration allows greater datatransfer between an array of memory cells and the microelectronic deviceusing, for example, conductive plugs formed within layers 112 and 150.

Electrodes 120 and 130 may be formed of any suitable conductivematerial. For example, electrodes 120 and 130 may be formed of dopedpolysilicon material or metal.

In accordance with one exemplary embodiment of the invention, one ofelectrodes 120 and 130 is formed of a material including a metal thatdissolves in ion conductor 140 when a sufficient bias (V∃V_(T)) isapplied across the electrodes (an oxidizable electrode) and the otherelectrode is relatively inert and does not dissolve during operation ofthe programmable device (an indifferent electrode). For example,electrode 120 may be an anode during a write process and be comprised ofa material including silver that dissolves in ion conductor 140 andelectrode 130 may be a cathode during the write process and be comprisedof an inert material such as tungsten, nickel, molybdenum, platinum,metal suicides, and the like. Having at least one electrode formed of amaterial including a metal which dissolves in ion conductor 140facilitates maintaining a desired dissolved metal concentration withinion conductor 140, which in turn facilitates rapid and stable region 160formation within ion conductor 140 or other electrical property changeduring use of structure 100 and/or 200. Furthermore, use of an inertmaterial for the other electrode (cathode during a write operation)facilitates electrodissolution of any region 160 that may have formedand/or return of the programmable device to an erased state afterapplication of a sufficient voltage.

During an erase operation, dissolution of any region 160 that may haveformed preferably begins at or near the oxidizable electrode/region 160interface. Initial dissolution of the region 160 at the oxidizableelectrode/region 160 interface may be facilitated by forming structure100 such that the resistance at the oxidizable electrode/region 160interface is greater than the resistance at any other point along region160, particularly, the interface between region 160 and the indifferentelectrode.

One way to achieve relatively low resistance at the indifferentelectrode is to form the electrode of relatively inert, non-oxidizingmaterial such as platinum. Use of such material reduces formation ofoxides at the interface between ion conductor 140 and the indifferentelectrode as well as the formation of compounds or mixtures of theelectrode material and ion conductor 140 material, which typically havea higher resistance than ion conductor 140 or the electrode material.

Relatively low resistance at the indifferent electrode may also beobtained by forming a barrier layer between the oxidizable electrode(anode during a write operation) and the ion conductor, wherein thebarrier layer is formed of material having a relatively high resistance.Exemplary high resistance materials are discussed in more detail below.

Reliable growth and dissolution of region 160 can also be facilitated byproviding a roughened indifferent electrode surface (e.g., a root meansquare roughness of greater than about 1 nm) at the electrode/ionconductor interface. The roughened surface may be formed by manipulatingfilm deposition parameters and/or by etching a portion of one of theelectrode or ion conductor surfaces. During a write operation,relatively high electrical fields form about the spikes or peaks of theroughened surface, and thus regions 160 are more likely to form aboutthe spikes or peaks. As a result, more reliable and uniform changes inelectrical properties for an applied voltage across electrodes 120 and130 may be obtained by providing a roughed interface between theindifferent electrode (cathode during a write operation) and ionconductor 140.

Oxidizable electrode material may have a tendency to thermally dissolveor diffuse into ion conductor 140, particularly during fabricationand/or operation of structure 100. The thermal diffusion is undesiredbecause it may reduce the resistance of structure 100 and thus reducethe change of an electrical property during use of structure 100.

To reduce undesired diffusion of oxidizable electrode material into ionconductor 140 and in accordance with another embodiment of theinvention, the oxidizable electrode includes a metal intercalated in atransition metal sulfide or selenide material such as A_(x)(MB₂)_(1-x),where A is Ag or Cu, B is S or Se, M is a transition metal such as Ta,V, and Ti, and x ranges from about 0.1 to about 0.7. The intercalatedmaterial mitigates undesired thermal diffusion of the metal (Ag or Cu)into the ion conductor material, while allowing the metal to participatein region 160 growth upon application of a sufficient voltage acrosselectrodes 120 and 130. For example, when silver is intercalated into aTaS₂ film, the TaS₂ film can include up to about 67 atomic percentsilver. The A_(x)(MB₂)_(1-x) material is preferably amorphous to preventundesired diffusion of the metal though the material. The amorphousmaterial may be formed by, for example, physical vapor deposition of atarget material comprising A_(x)(MB₂)_(1-x).

α-AgI is another suitable material for the oxidizable electrode. Similarto the A_(x)(MB₂)_(1-x) material discussed above, α-AgI can serve as asource of Ag during operation of structure 100—e.g., upon application ofa sufficient bias, but the silver in the AgI material does not readilythermally diffuse into ion conductor 140. AgI has a relatively lowactivation energy for conduction of electricity and does not requiredoping to achieve relatively high conductivity. When the oxidizableelectrode is formed of AgI, depletion of silver in the AgI layer mayarise during operation of structure 100, unless excess silver isprovided to the electrode. One way to provide the excess silver is toform a silver layer adjacent the AgI layer. When interposed between alayer of silver and ion conductor 140, the AgI layer reduces thermaldiffusion of Ag into ion conductor 140, but does not significantlyaffect conduction of Ag during operation of structure 100. In addition,use of AgI increases the operational efficiency of structure 100 becausethe AgI mitigates non-Faradaic conduction (conduction of electrons thatdo not participate in the electrochemical reaction).

In accordance with one embodiment of the invention, at least oneelectrode 120 and 130 is formed of material suitable for use as aninterconnect metal. For example, electrode 130 may form part of aninterconnect structure within a semiconductor integrated circuit. Inaccordance with one aspect of this embodiment, electrode 130 is formedof a material that is substantially insoluble in material comprising ionconductor 140. Exemplary materials suitable for both interconnect andelectrode 130 material include metals and compounds such as tungsten,nickel, molybdenum, platinum, metal silicides, and the like.

As noted above, programmable structures of the present invention mayinclude one or more barrier or buffer layers 155, 255 interposed betweenat least a portion of ion conductor 140 and one of the electrodes 120,130. Layers 155, 255 may include ion conductors such as Ag_(x)O,Ag_(x)S, Ag_(x)Se, Ag_(x)Te, where x∃2, Ag_(y)I, where x∃1, CuI₂, CuO,CuS, CuSe, CuTe, GeO₂, Ge_(z)S_(1-z), Ge_(z)Se_(1-z), Ge_(z)Te_(1-z),As_(z)S_(1-z), As_(z)Se_(1-z), As_(z)Te_(1-z), where z is greater thanor equal to about 0.1, SiO_(x), and combinations of these materials)interposed between ion conductor 140 and a metal layer such as silver.

Other materials suitable for buffer layers 155 and/or 255 include GeO₂.Amorphous GeO₂ is relatively porous an will “soak up” silver duringoperation of device 100, but will retard the thermal diffusion of silverto ion conductor 140, compared to structures or devices that do notinclude a buffer layer. When ion conductor 140 includes germanium, GeO₂may be formed by exposing ion conductor 140 to an oxidizing environmentat a temperature of about 300° C. to about 800° C. or by exposing ionconductor 140 to an oxidizing environment in the presence of radiationhaving an energy greater than the band gap of the ion conductormaterial. The GeO₂ may also be deposited using physical vapor deposition(from a GeO₂ target) or chemical vapor deposition (from GeH₄ and an O₂).

Buffer layers can also be used to increase the off resistance and “writevoltage” by placing a high-resistance buffer layer (e.g., GeO₂, SiO_(x),air, a vacuum, or the like) between ion conductor 140 and theindifferent electrode. In this case, the high-resistance buffer materialallows metal such as silver to diffuse though or plate across the bufferand take part in the electrochemical reaction.

When the barrier layer between the indifferent electrode and the ionconductor includes a high resistance material, the barrier may includeions that contribute to electrodeposit growth or the barrier may bedevoid of ions. In either case, the barrier must be able to transmitelectrons, by conduction or tunneling, such that the redox reactionoccurs, allowing for region 160 growth.

In some cases, an electrodeposit may form within the high-resistancebarrier layer. Exemplary high-resistance barrier layers that supportelectrodeposit growth include gas-filled or vacuum gap regions, porousoxide films, of other high-resistance glassy materials, andsemiconductor material as long as the barrier is thin enough to allowelectron tunneling from the cathode to the ion conductor at reasonablevoltages (e.g., less than or equal to about 1 volt), can supportelectron transport, and can allow ions to be reduced within the barriermaterial volume.

Layers 155 and/or 255 may also include a material that restrictsmigration of ions between conductor 140 and the electrodes. Inaccordance with exemplary embodiments of the invention, a barrier layerincludes conducting material such as titanium nitride, titaniumtungsten, a combination thereof, or the like. The barrier may beelectrically indifferent, i.e., it allows conduction of electronsthrough structure 100 or 200, but it does not itself contribute ions toconduction through structure 200. An electrically indifferent barriermay reduce undesired electrodeposit growth during operation of theprogrammable device, and thus may facilitate an “erase” or dissolutionof region 160 when a bias is applied which is opposite to that used togrow region 160. In addition, use of a conducting barrier allows for the“indifferent” electrode to be formed of oxidizable material because thebarrier prevents diffusion of the electrode material to the ionconductor.

Ion conductor 140 is formed of material that conducts ions uponapplication of a sufficient voltage. Suitable materials for ionconductor 140 include glasses and semiconductor materials. In general,ion conductors in accordance with the present invention can conduct ionswithout requiring a phase change, can conduct ions at a relatively lowtemperature (e.g., below 125° C.), can conduct ions at relatively lowelectrical currents, have a relatively high transport number, andexhibit relatively high ion conductivity. In one exemplary embodiment ofthe invention, ion conductor 140 is formed of chalcogenide material.However, other materials may be used as an ion conductor in accordancewith various embodiments of the invention.

Ion conductor 140 may also suitably include dissolved conductivematerial. For example, ion conductor 140 may comprise a solid solutionthat includes dissolved metals and/or metal ions. In accordance with oneexemplary embodiment of the invention, conductor 140 includes metaland/or metal ions dissolved in chalcogenide glass. An exemplarychalcogenide glass with dissolved metal in accordance with the presentinvention includes a solid solution of As_(x)S_(1-x)—Ag,As_(x)Se_(1-x)—Ag, As_(x)Te_(1-x)—Ag, Ge_(x)Se_(1-x)—Ag,Ge_(x)S_(1-x)—Ag, Ge_(x)Te_(1-x)—Ag, As_(x)S_(1-x)—Cu,As_(x)Se_(1-x)—Cu, As_(x)Te_(1-x)—Cu, Ge_(x)Se_(1-x)—Cu,Ge_(x)S_(1-x)—Cu, and Ge_(x)Te_(1-x)—Cu where x ranges from about 0.1 toabout 0.5, other chalcogenide materials including silver, copper,combinations of these materials, and the like. In addition, conductor140 may include network modifiers that affect mobility of ions throughconductor 140. For example, materials such as metals (e.g., silver),halogens, halides, or hydrogen may be added to conductor 140 to enhanceion mobility and thus increase erase/write speeds of the structure.Furthermore, as discussed in more detail below, ion conductor 140 mayinclude a plurality of regions having different resistance values—forexample, ion conductor 140 may include a first region proximate theoxidizable electrode having a relatively low resistance and a secondregion proximate the indifferent electrode having a relatively highresistance.

To increase the thermal stability of ion conductor, doped oxides and/oroxide-doped chalcogenides are used as ion conductor 140. Exemplary oxidedopants for chalcogenide materials include oxygen, GeO₂, As₂O₃, Ag₂O,Cu_((1,2))O, and SiO₂ and exemplary oxides suitable for doping includesilver or copper doped GeO₂, As₂O₃, Ag₂O, CU_((1,2))O, WO_(x) and othertransition metal oxides and SiO_(x). In the case of doped oxides, ionconductor 140 is preferably less than about 10 nm thick.

Ion conductor 140 may also include a filler material, which fillsinterstices or voids. Suitable filler materials include non-oxidizableand non-silver based materials such as a non-conducting, immisciblesilicon oxide and/or silicon nitride, having a cross-sectional dimensionof less than about 1 nm, which do not contribute to the growth of region160. In this case, the filler material is present in the ion conductorat a volume percent of up to about 5 percent to reduce a likelihood thata region 160 will spontaneously dissolve into the supporting ternarymaterial as the device is exposed to elevated temperature, which leadsto more stable device operation without compromising the performance ofthe device. Ion conductor 140 may also include filler material to reducean effective cross-sectional area of the ion conductor. In this case,the concentration of the filler material, which may be the same fillermaterial described above but having a cross-sectional dimension up toabout 50 nm, is present in the ion conductor material at a concentrationof up to about 50 percent by volume.

In accordance with one exemplary embodiment of the invention, ionconductor 140 includes a germanium-selenide glass with silver diffusedin the glass. Germanium selenide materials are typically formed fromselenium and Ge(Se)_(4/2) tetrahedra that may combine in a variety ofways. In a Se-rich region, Ge is 4-fold coordinated and Se is 2-foldcoordinated, which means that a glass composition nearGe_(0.20)Se_(0.80) will have a mean coordination number of about 2.4.Glass with this coordination number is considered by constraint countingtheory to be optimally constrained and hence very stable with respect todevitrification. The network in such a glass is known to self-organizeand become stress-free, making it easy for any additive, e.g., silver,to finely disperse and form a mixed-glass solid solution. Accordingly,in accordance with one embodiment of the invention, ion conductor 140includes a glass having a composition of Ge_(0.17)Se_(0.83) toGe_(0.25)Se_(0.75).

When conductive material such as metal is added to an ion conductormaterial, phase-separated regions of the metal-doped ion conductor mayform. In this case, a macroscopic view of the doped ion conductor mayappear glassy even though small, phases-separated regions are formed.

FIGS. 3-5 illustrate portions of programmable structures, which includephase-separated ion conductor material, in accordance with variousembodiments of the present invention. FIG. 3 illustrates aphase-separated ion conductor region 302, including a high-resistanceportion 304 and low-resistance portions 306. By way of particularexample, when ion conductor 302 includes silver doped Ge_(x)Se_(1-x),where x is less than about 0.33 and preferably ranges from about 0.17 toabout 0.3 and more preferably has a value of about 0.17 to about 0.25,ion conductor separates into a first phase 304 of Ge—Se (e.g., Ge₂Se₃)having a relatively high resistivity of greater than about 10³ ohm-cmand a second phase 306 of Ag₂Se, which is much more conductive thatGe—Se portion 304, having an ion resistivity of about 0.3 ohm-cm and anion mobility of about 10⁻⁴ cm²/V.s. In this case, phase-separated ionconductor 302 has an overall resistivity of about 100 ohm-cm and isstable at room temperature.

It is thought that phase-separated ion conductors facilitate large offresistance and high switching speed of programmable devices such asdevice 100. The reason for this is that the metal ions from the solubleelectrode will migrate within region 304 to bridge low-resistanceregions 306. Reduction of metal ions preferentially occurs inhigh-resistance regions 304 because the local field is highest in thisarea of ion conductor 302. This process is relatively fast because atypical gap between low-resistance portions is on the order of about 1nm or less.

Other exemplary materials suitable for phase-separated ion-conductormaterial include silver and/or copper-doped germanium chalcogenides(e.g., sulfides and tellurides) and mixtures of these compounds, silverand/or copper-doped arsenic chalcogenides (e.g., selenides, sulfides,and tellurides) and mixtures of these compounds. Other exemplaryphase-separate ion conductors include Ag₂Se dispersed within AgI orwithin an ion conductive polymer such as poly(ethylene oxide) andadditional exemplary low-resistance material suitable for portion 304include SiO_(x), GeO₂, and Ag₂O. It should be noted, however, that anyion conductor material that includes a low-resistance phase dispersedwithin a low-resistance phase will function in accordance with thepresent invention as described herein.

FIG. 4 illustrates a structure 400, including a phase-separated ionconductor 402. Ion conductor 402 includes high-resistance portions 404and low-resistance portions 406. Portions 404 and 406 may be formed ofcorresponding high-resistance and low-resistance material describedabove in connection with portions 304 and 306. Structure 400 may beformed by sequentially depositing high-resitivity material andlow-resistance material. Although illustrated with only twohigh-resistance portions and two low-resistance portions, structures inaccordance with the present invention may include any desired number oflow and high-resistivity portions.

Similar to the operation of structure 300, the operation speed ofstructure 400 is primarily limited by the change of resistance ofhigh-resistance portions 404. Accordingly, operational speed ofstructures 400 can be manipulated by altering a thickness of layer(s)404.

FIG. 5 illustrates another structure 500, which includes phase separatedion conductor material 502. Structure 500 is similar to structures 300and 400, except that structure 500 includes particles 508, including ahigh-resistance portion 504 and a low-resistance portion 506, which maybe formed of low and high-resistance materials described herein.

Referring again to FIGS. 1 and 2, in accordance with one exemplaryembodiment of the invention, at least a portion of structure 100 isformed within a via of an insulating material 150. Forming a portion ofstructure 100 within a via of an insulating material 150 may bedesirable because, among other reasons, such formation allows relativelysmall structures, e.g., on the order of 10 nanometers, to be formed. Inaddition, insulating material 150 facilitates isolating variousstructures 100 from other electrical components.

Insulating material 150 suitably includes material that preventsundesired diffusion of electrons and/or ions from structure 100. Inaccordance with one embodiment of the invention, material 150 includessilicon nitride, silicon oxynitride, polymeric materials such aspolyimide or parylene, or any combination thereof.

A contact 165 may suitably be electrically coupled to one or moreelectrodes 120, 130 to facilitate forming electrical contact to therespective electrode. Contact 165 may be formed of any conductivematerial and is preferably formed of a metal, alloy, or compositionincluding aluminum, tungsten, or copper.

In accordance with one embodiment of the invention, structure 100 isformed by forming electrode 130 on substrate 110. Electrode 130 may beformed using any suitable method such as, for example, depositing alayer of electrode 130 material, patterning the electrode materials andetching the material to form electrode 130. Insulating layer 150 may beformed by depositing insulating material onto electrode 130 andsubstrate 110 and forming vias in the insulating material usingappropriate patterning and etching processes. Ion conductor 140 andelectrode 120 may then be formed within insulating layer 150 bydepositing ion conductor 140 material and electrode 120 material withinthe via. Such ion conductor and electrode material deposition may beselective—i.e., the material is substantially deposited only within thevia, or the deposition processes may be relatively non-selective. If oneor more non-selective deposition methods are used, any excess materialremaining on a surface of insulating layer 150 may be removed using, forexample, chemical mechanical polishing and/or etching techniques.Barrier layers 155 and/or 255 may similarly be formed using any suitabledeposition and/or etch processes.

A solid solution suitable for use as ion conductor 140 may be formed ina variety of ways. For example, the solid solution may be formed bydepositing a layer of conductive material such as metal over achalcogenide glass without breaking a vacuum and exposing the metal andglass to thermal and/or photo dissolution processing. In accordance withone exemplary embodiment of the invention, a solid solution of As₂S₃—Agis formed by depositing As₂S₃ onto a substrate, depositing a thin filmof Ag onto the As₂S₃, and exposing the films to light having energygreater than the optical gap of the As₂S₃,—e.g., light having awavelength of less than about 500 nanometers (e.g., light having awavelength of about 436 nm at about 6.5 mW/cm²). With this process thechalcogenide glass can incorporate over 30 atomic percent of silver andremain macroscopically glassy and microscopically phase separated. Ifdesired, network modifiers may be added to conductor 140 duringdeposition of conductor 140 (e.g., the modifier is in the depositedmaterial or present during conductor 140 material deposition) or afterconductor 140 material is deposited (e.g., by exposing conductor 140 toan atmosphere including the network modifier).

In accordance with another embodiment of the invention, a solid solutionmay be formed by depositing one of the constituents from a source onto asubstrate or another material layer and reacting the first constituentwith a second constituent. For example, germanium (preferably amorphous)may be deposited onto a portion of a substrate and the germanium may bereacted with H₂Se to form a Ge—Se glass. Similarly, arsenic can bedeposited and reacted with the H₂Se gas, or arsenic or germanium can bedeposited and reacted with H₂S gas. Silver or other metal can then beadded to the glass as described above.

When used, oxides may be added to the ion conductor material by addingan oxide to a melt used to form a chalcogenide ion conductor source. Forexample, GeO₂, As₂O₃, Ag₂O, Cu_((1,2))O, and SiO₂, can be added toGe_(x)S_(1-x), As_(x)S_(1-x), Ge_(x)Se_(1-x), As_(x)Se_(1-x),Ge_(x)Te_(1-x), As_(x)Te_(1-x), to form an oxide-chalcogenide glassincluding up to several tens of atomic percent oxygen. The ternary orquaternary glass can then be used to deposit a film of similarcomposition on the device substrate by physical vapor deposition orsimilar technique. Alternatively, the oxygen-containing film may beformed in-situ using reactive deposition techniques in which thechalcogenide material is deposited in a reactive oxygen ambient to forman ion conductor including up to several tens of atomic percent of boundoxygen. Conductive material such as silver or copper can be incorporatedinto the source glass melt or introduced into the deposited film bythermal or photo-dissolution as discussed above.

Similarly, metal doped oxides may be deposited from a synthesized sourcewhich contains all the necessary elements in the correct proportions(e.g., Ag_(x)O (x>2), Cu_(x)O (x>2), Ag/Cu—GeO₂, Ag/Cu—As₂O₃, orAg/Cu—SiO₂) or the silver or copper may be introduced into the binaryoxide film (Ag₂O, CU_((1,2))O, GeO₂, As₂O₃, or SiO₂) by thermal- orphoto-dissolution from a thin surface layer of the metal. Alternatively,a base layer of Ag, Cu, Ge, As, or Si may be deposited first and thenreacted with oxygen to form the appropriate oxide and then diffused withAg or Cu as discussed above. The oxygen reaction could be purely thermalor plasma-assisted, the latter producing a more porous oxide.

One of the electrodes may be formed during ion conductor 140 doping bydepositing sufficient metal onto an ion conductor material and applyingsufficient electrical or thermal energy to the layers such that aportion of the metal is dissolved within the ion conductor material anda portion of the metal remains on a surface of the ion conductor to forman electrode (e.g., electrode 120). Regions of differing conductivitywithin ion conductor 140 can be formed using this technique by applyinga sufficient amount of energy to the structure such that a first portionof the ion conductor proximate the soluble electrode contains a greateramount of conductive material than a second portion of the ion conductorproximate the indifferent electrode. This process is self limiting ifion starting ion conductor layer is thick enough so that a portion ofthe film becomes saturated and a portion of the film is unsaturated.

In accordance with alternative embodiments of the invention, solidsolutions containing dissolved metals may be directly deposited ontosubstrate 110 and the electrode then formed overlying the ion conductor.For example, a source including both chalcogenide glass and conductivematerial can be used to form ion conductor 140 using physical vapordeposition or similar techniques.

An amount of conductive material such as metal dissolved in an ionconducting material such as chalcogenide may depend on several factorssuch as an amount of metal available for dissolution and an amount ofenergy applied during the dissolution process. However, when asufficient amount of metal and energy are available for dissolution inchalcogenide material using photodissolution, the dissolution process isthought to be self limiting, substantially halting when the metalcations have been reduced to their lowest oxidation state. In the caseof As₂S₃—Ag, this occurs at Ag₄As₂S₃=2Ag₂S+As₂S, having a silverconcentration of about 47 atomic percent. If, on the other hand, themetal is dissolved in the chalcogenide material using thermaldissolution, a higher atomic percentage of metal in the solid solutionmay be obtained, provided a sufficient amount of metal is available fordissolution.

In accordance with a further embodiment of the invention, the solidsolution is formed by photodissolution to form a macrohomogeneousternary compound and additional metal is added to the solution usingthermal diffusion (e.g., in an inert environment at a temperature ofabout 85° C. to about 150° C.) to form a solid solution containing, forexample, about 30 to about 50, and preferably about 34 atomic percentsilver. Ion conductors having a metal concentration above thephotodissolution solubility level facilitates formation of regions 160that are thermally stable at operating temperatures (typically about 85°C. to about 150° C.) of devices 100 and 200. Alternatively, the solidsolution may be formed by thermally dissolving the metal into the ionconductor at the temperature noted above; however, solid solutionsformed exclusively from photodissolution are thought to be lesshomogeneous than films having similar metal concentrations formed usingphotodissolution and thermal dissolution.

Information may be stored using programmable structures of the presentinvention by manipulating one or more electrical properties of thestructures. For example, a resistance of a structure may be changed froma “0” or off state to a “1” or on state during a suitable writeoperation. Similarly, the device may be changed from a “1” state to a“0” state during an erase operation. In addition, as discussed in moredetail below, the structure may have multiple programmable states suchthat multiple bits of information are stored in a single structure.

Write Operation

FIG. 6 illustrates current-voltage characteristics of a programmablestructure (e.g. structure 200) in accordance with the present invention.In the illustrated embodiment, via diameter, D, is about 4 microns,conductor 140 is about 35 nanometers thick and formed of Ge₃Se₇—Ag (nearAg₈Ge₃Se₇), electrode 130 is indifferent and formed of nickel, electrode120 is formed of silver, and barrier 255 is a native nickel oxide. Asillustrated in FIG. 6, current through structure 200 in an off state(curve 610) begins to rise upon application of a bias of over about onevolt; however, once a write step has been performed (i.e., anelectrodeposit has formed), the resistance through conductor 140 dropssignificantly (i.e., to about 200 ohms), illustrated by curve 620 inFIG. 6. As noted above, when electrode 130 is coupled to a more negativeend of a voltage supply, compared to electrode 120, a conductive regionbegins to form near electrode 130 and grow toward electrode 120. Aneffective threshold voltage (i.e., voltage required to cause growth ofthe conductive region and to break through barrier 255, thereby couplingelectrodes 120, 130 together) is relatively high because of barrier 255.In particular, a voltage V∃V_(T) must be applied to structure 200sufficient to cause electrons to tunnel through barrier 255 (whenbarrier 255 comprises an insulating layer) to form the conductive regionand to overcome the barrier (e.g., by tunneling through or leakage) andconduct through conductor 140 and at least a portion of barrier 255.

In accordance with alternate embodiments of the invention, where noinsulating barrier layer is present, an initial “write” thresholdvoltage is relatively low because no insulative barrier is formedbetween, for example, ion conductor 140 and either of the electrodes120, 130.

As noted above, the relative volatility of the memory structures of thepresent invention may be altered by applying different amounts of energyto the structures during a write process. For example, a relatively highcurrent pulse of a few hundred microamperes for a period of aboutseveral hundred nanoseconds may be applied to the structures illustratedin FIGS. 1 and 2 to form a relatively nonvolatile memory cell.Alternatively, the same current may be supplied to the same or similarmemory structure for a shorter amount of time, e.g., several nanosecondsto form a relatively volatile memory structure. In either case, thememory of the present invention can be programmed at relatively highspeeds and even the “volatile” memory is relatively nonvolatile comparedto traditional DRAM. For example, the volatile memory may operate atspeed comparable to DRAM and only require refreshing every severalhours.

Read Operation

A state of a memory cell (e.g., 1 or 0) may be read, withoutsignificantly disturbing the state, by, for example, applying a forwardor reverse bias of magnitude less than a voltage threshold (about 1.4 Vfor a structure illustrated in FIG. 6) for electrodeposition or by usinga current limit which is less than or equal to the minimum programmingcurrent (the current which will produce the highest of the on resistancevalues). A current limited (to about 1 milliamp) read operation isillustrated in FIG. 6. In this case, the voltage is swept from 0 toabout 2 V and the current rises up to the set limit (from 0 to 0.2 V),indicating a low resistance (ohmic/linear current-voltage) “on” state.Another way of performing a non-disturb read operation is to apply apulse, with a relatively short duration, which may have a voltage higherthan the electrochemical deposition threshold voltage such that noappreciable Faradaic current flows, i.e., nearly all the current goes topolarizing/charging the device and not into the electrodepositionprocess.

In accordance with various embodiments of the invention, circuitsincluding the programmable structures include temperature compensationdevices to mitigate effects of temperature variation on the performanceof the programmable device. One exemplary temperature compensationcircuit includes a programmable structure having a known erased state.In this case, during a read operation, a progressively increasingvoltage is applied to a programmable structure having an unknown stateas well as to the structure having the known erased state. If theunknown structure has been written to, it will switch on before theknown erased device and if the unknown structure is in an erased state,the two devices will switch on at approximately the same time.Alternatively, a temperature compensation circuit can be used to producea comparison voltage or current to be compared to a voltage or currentproduced by a programmable structure of an unknown state during a readprocess.

Erase Operation

A programmable structure (e.g., structure 200) may suitably be erased byreversing a bias applied during a write operation, wherein a magnitudeof the applied bias is equal to or greater than the threshold voltagefor electrodeposition in the reverse direction. In accordance with anexemplary embodiment of the invention, a sufficient erase voltage(V∃V_(T)) is applied to structure 200 for a period of time, whichdepends on energy supplied during the write operation, but is typicallyless than about 1 millisecond to return structure 200 to its “off” statehaving a resistance well in excess of a million ohms. In cases where theprogrammable structure does not include a barrier between conductor 140and electrode 120, a threshold voltage for erasing the structure is muchlower than a threshold voltage for writing the structure because, unlikethe write operation, the erase operation does not require electrontunneling through a barrier or barrier breakdown.

Pulse Mode Read/Write

In accordance with an alternate embodiment of the invention, pulse modeprogramming is used to write to and read from a programmable structure.In this case, similar to the process described above, region 160 formsduring a write process; however, unlike the process described above, atleast a portion region 160 is removed or dissolved during a readoperation. During an erase/read process, the magnitude of the currentpulse is detected to determine the state (1 or 0) of the device. If thedevice had not previously been written to or has previously been erased,no ion current pulse will be detected at or above thereduction/oxidation potential of the structure. But, if the device is ina written state, an elevated current will be detected during thedestructive read/erase step. Because this is a destructive readoperation, information must be written to each structure after each readprocess—similar to DRAM read/write operations. However, unlike DRAMdevices, the structures of the present invention are stable enough toallow a range of values to be stored (e.g., various amounts of region160). Thus, a partially destructive read that decrease, but does notcompletely eliminate region 160, can be used. In accordance with analternate aspect of this embodiment, a destructive write process ratherthan a destructive erase process can be used read the device. In thiscase, if the cell is in an “off” state, a write pulse will produce anion current spike as region 160 forms, whereas a device that alreadyincludes a region 160 will not produce the ion current spike if theprocess has been limited by a lack of oxidizable silver.

FIG. 7 illustrates an array 700 of programmable structure 702-708 thatform rows R₁-R₃ and column C₁-C₃ of programmable structures. Whenpulse-mode programming is employed to read and write to array 700,addition diodes and transistors typically used for structure isolation,are not required, so long as regions 160 of the structures are not grownto an extent that they short the structure. A non-bridging region can beobtained by using limited write times, limiting an amount of oxidazablematerial at the oxidizable electrode that can contribute to region 160formation, and/or using a resistive region between the electrodes thatallows sufficient electron current, but prevents or decreaseselectrodeposition throughout ion conductor 140.

In array 700, a non-bridging region 160 is grown in the selectedstructure by, for example, biasing C₂=+V_(t)/2, R₂=−V_(t)/2, where V_(t)is the reduction/oxidation potential, with all other rows and columnsheld at or near zero volts, so that no other device in the array seesthe full forward write voltage. The resistance remains high afterwriting as the electrodeposit (region 160) does not bridge theelectrodes. The read/erase bias is C₂=−V_(t)/2, R₂=+V_(t)/2 with allothers held at zero volts. If the device is in the off state and theerase pulse is applied, very little current will flow through the highresistance electrolyte. If, however, a partial electrodeposit has beenformed by a write operation, an erase pulse will produce an ion currentspike through C₂ and R₂ as the electrodeposited metal is oxidized andre-plated on the negative electrode (the oxidizable anode in the writeprocess). This current spike can be sensed and therefore the state ofthe selected cell can be determined by this process. Note that it isalso possible to use full rather than half or partial voltages when thenon-selected rows and columns are allowed to float (via the use oftri-state drivers); e.g., C₂=+V_(t) and R₂=0 for write, C₂=0 andR₂=+V_(t) for read/erase.

Control of Operational Parameters

The concentration of conductive material in the ion conductor can becontrolled by applying a bias across the programmable device. Forexample, metal such as silver may be taken out of solution by applying anegative voltage in excess of the reduction potential of the conductivematerial. Conversely, conductive material may be added to the ionconductor (from one of the electrodes) by applying a bias in excess ofthe oxidation potential of the material. Thus, for example, if theconductive material concentration is above that desired for a particulardevice application, the concentration can be reduced by reverse biasingthe device to reduce the concentration of the conductive material.Similarly, metal may be added to the solution from the oxidizableelectrode by applying a sufficient forward bias. Additionally, it ispossible to remove excess metal build up at the indifferent electrode byapplying a reverse bias for an extended time or an extended bias overthat required to erase the device under normal operating conditions.Control of the conductive material may be accomplished automaticallyusing a suitable microprocessor.

With particular reference to FIGS. 3-5, a partial write or a partialerase caused by a forward or reverse programming pulse of insufficientduration and/or current to introduce enough silver to significantlyreduce the resistance of the high-resistance regions will causeadditional silver to migrate from the soluble electrode thehigh-resistance portions. The conductivity and activation energy inthese zones are altered by this excess silver even though the devicewill appear to be in a high resistance state. The change in activationenergy results in higher ion mobility and hence this partialpreprogramming results in much faster switching than in a device whichhad been fully written or erased, which in turn allows appropriatelyscaled devices to operate at SRAM speeds (in the nanosecond or lessrange).

In addition, in a written device that has “faded” due to thermaldiffusion of the electrodeposited material away from the low resistancepathway, the local excess silver will still promote a lower activationenergy but in this case, there is also sufficient silver present thatthe voltage required to reform the conducting link will be lower than inthe case of a normal write. The “reclosure” voltage will be several tensof mV lower than the normal write threshold potential of the systemsince the silver required to close the link is already in the materialand does not have to be released from the oxidizable electrode. Thismeans that a simple “read” operation involving a short pulse below thewrite threshold potential will be sufficient to regenerate a fadedon-state in the device but will be insufficient to disturb a device thathas been fully erased (and hence does not have the excess silver). Thiswill allow faded lightly written (low programming current) states to beautomatically regenerated by the read operation, thereby extending theeffective retention of the devices.

This technique may also be used to form one of the electrodes frommaterial within the ion conductor material. For example, silver from theion conductor may be plated out to form the oxidizable electrode. Thisallows the oxidizable electrode to be formed after the device is fullyformed and thus mitigates problems associated with conductive materialdiffusing from the oxidizable electrode during manufacturing of thedevice.

The threshold voltage of programmable devices may be manipulated inaccordance with various embodiments of the present invention.Manipulation of the threshold voltage allows configuration of theprogrammable devices for desired read and write voltages. In general, asnoted above, the threshold voltage depends on, among other things, anamount of conductive material present in the ion conductor and/or anybarrier.

One way to manipulate the electrodeposition threshold voltage is tomanipulate the conductive material dispersed within the ion conductormaterial. Another technique for manipulating the threshold voltage is toalter an amount of oxidizable material at or near the indifferentelectrode. In this case, the oxidizable metal at the cathode can bealtered by first forming an electrodeposit at or near the indifferentelectrode and then applying a reverse bias sufficient to dissolve aportion of the electrodeposit. The threshold voltage generally goes downas the amount of oxidizable metal at the cathode goes up. For example,in the case where the ion conductor is Ge_(0.3)Se_(0.7) and the solubleelectrode is silver, the threshold voltage for electrodeposit formationis about 310±10 mV for no predeposited silver to about 90±10 mV for asilver saturated electrode. Alternatively, a write process may be usedto form a desired electrodeposit at or near the cathode. Thiselectrochemical control of the threshold voltage can be used to heal orregenerate an electrodeposit that has been thermally orelectrochemically damaged or redistributed. As an example of how thiswould work, consider the following:

1. An electrodeposit can be formed in or on the electrolyte using awrite voltage that is determined by the Ag concentration near thecathode, e.g., 0.32 V for a silver depleted cathode region.

2. A read voltage below the write voltage may be used to determine thestate of the device without disturbing an off device.

3. The electrodeposit is subsequently “damaged” by thermal diffusion(e.g., excessive external device heating) so that the electrodeposit isno longer continuous or localized.

4. The Ag concentration near the cathode will still be higher than inthe case of an unwritten or completely erased device as the silvercannot diffuse against the diffusion gradient.

5. The increased cathodic silver results in a reduction of the re-writevoltage, e.g., below the read voltage which is chosen not to disturb anoff state but to be high enough to “regenerate” failed electrodeposits.

Therefore, a read operation can be used to automaticallyre-electrodeposit the silver and regenerate the data state stored in thedevice via the electrodeposited material. This will ensure that theprogrammable structures effectively retain data for extended periods oftime.

Another technique for manipulating threshold voltage is by forming alayer of material between the indifferent electrode and the ionconductor layer, wherein the layer of material is capable of conductingions and forming an electrodeposit and has a lower concentration ofoxidizable material than the ion conductor. Exemplary materials suitablefor this layer include undoped or lightly doped chalcogenide materialssuch as Ge—S or Ge—Se and a variety of other undoped or lightly dopedion conductor materials that have a low solubility level for theoxidizable material. The material layer between the indifferentelectrode and the ion conductor reduces oxidizable material build upneat the indifferent electrode during processing and operation of theprogrammable structure.

An example of a write operation on a device which includes a silversaturated sulfur-rich Ge—S starting glass and a layer of relatively lowsilver content, less than 10 nm thick, between the indifferent electrodeand the saturated ion conductor is illustrated in FIG. 8 (current vs.voltage for a 1 μA programming current) and FIG. 9 (resistance vs.voltage for a 1 μA programming current). In the illustrated case, thewrite voltage lies around 320 mV. Note that this layer could bedeposited separately from the Ag-rich electrolyte or can be formed, asdescribed above, by stopping the photodiffusion before the oxidazablemetal penetrates the entire film thickness.

To mitigate undesired diffusion of an electrodeposit formed during awrite process, it may be desirable to saturate the ion conductor withconductive material during a write process, such that the resistance ofthe structure does not substantially change due to diffusion ofconductive material. In the case where the ion conductor comprisesgermanium selenide doped with silver and the electrodeposit is formedfrom silver, the approximate minimum “saturation” programming current tomaintain electrodeposition until the electrolyte has a uniform silverconcentration and corresponding resistance as a function of electrolytearea is given below for a 10 nm thick silver-doped germanium selenidesolid electrolyte.

Area (nm/nm²) Saturation current (μA) Resistance (kΩ) 1 0.3 1,000 10 3100 100 30 10 1000 300 1 10,000 3,000 0.1

FIGS. 10 and 11 illustrate additional structures, which are configuredto mitigate any unwanted diffusion of conductive material within an ionconductor, in accordance with further exemplary embodiments of thepresent invention. In general, the structures illustrated in FIGS. 10and 11 include a relatively narrow ion conductor (compared to the heightof the ion conductor) surrounded by a material which is less ionconductive than the ion conductor.

Structure 900 includes a first electrode 902, an insulating layer 904, adiffusion barrier 906, an ion conductor 908 and a second electrode 910.Electrodes 902 and 910, insulating layer 904, and ion conductor 908 maybe formed of the corresponding materials and using the same techniquesdescribed above in connection with FIGS. 1 and 2. The barriers may beconfigured such that only one “column” of ion conductor material spansbetween the electrodes or the structures may include a plurality of ionconductor columns that span between the electrodes. In the latter case,the electrodeposition may only occur in one of the columns if thecurrent is appropriately limited. In either case, because the columndiameter is relatively small (e.g., less than about 50 nm) the currentrequired to saturate the region is also relatively small (e.g., about 30μA)

Diffusion barrier 906 can be formed by conformally depositing a barriermaterial 1002 such as silicon nitride or any of the barrier materialsdiscussed above in connection with layer 155 and 255 and illustrated inFIG. 11 and removing a portion of the barrier material (e.g., using ananisotropic etch) to form barrier 906 illustrated in FIG. 10. Ionconductor 908 can then be formed using deposition and etch or damascenetechniques.

Another technique for forming diffusion barriers is illustrated in FIGS.12 and 13. In this case, porous barrier material 1102 is formed on asurface of a substrate 1104 and ion conductor material is formed in theporous regions of barrier material 1102. Alternatively, porous material1102 can be used as an etch mask to etch ion conductor material (usingan anisotropic etch) to form columns of ion conductor material. Spacesbetween to columns are then filled with appropriate barrier maters suchas silicon nitride. Electrodes may be formed about one or more columnsof ion conductor material as described herein to form programmabledevices of the present invention.

FIG. 14 illustrates yet another structure 1300 suitable for formingdiffusion-resistant programmable structures. Structure 1300 includes afirst electrode 1302, a second electrode 1304, an ion conductor 1306,and a material layer 1308 that favors electrodeposit growthsubstantially in only one direction. Electrodes 1302 and 1304 and ionconductor 1306 may be formed of any of the corresponding electrodes andion conductors described herein. Suitable exemplary materials formaterial layer 1308 include thin (e.g. a few nm) amorphous films ofoxides and nitrides (e.g., SiO₂, GeO₂, Si₃N₄) and semiconductors (Si,Ge) and polycrystalline films of oxides, nitrides, and semiconductors.In accordance with one aspect of this embodiment of the invention, layer1308 is formed between ion conductor 1306 and the indifferent electrode.

As noted above, in accordance with yet another embodiment of theinvention, multiple bits of data may be stored within a singleprogrammable structure by controlling a size of region 160 which isformed during a write process. A size of region 160 that forms during awrite process depends on a number of coulombs or charge supplied to thestructure during the write process, and may be controlled by using acurrent limit power source. In this case, a resistance of a programmablestructure is governed by Equation 1, where R_(on) is the “on” stateresistance, V_(T) is the threshold voltage for electrodeposition , andI_(LIM) is the maximum current allowed to flow during the writeoperation.

$\begin{matrix}{R_{on} = \begin{matrix}V_{T} \\{XXXX} \\I_{LIM}\end{matrix}} & {{Equation}\mspace{20mu} 1}\end{matrix}$

In practice, the limitation to the amount of information stored in eachcell will depend on how stable each of the resistance states is withtime. For example, if a structure with a programmed resistance range ofabout 3.5 kΩ and a resistance drift over a specified time for each stateis about ±250Ω, about 7 equally sized bands of resistance (7 states)could be formed, allowing 3 bits of data to be stored within a singlestructure. In the limit, for near zero drift in resistance in aspecified time limit, information could be stored as a continuum ofstates, i.e., in analog form.

A portion of an integrated circuit 1402, including a programmablestructure 1400, configured to provide additional isolation fromelectronic components is illustrated in FIG. 15. Structure 1400 includeselectrodes 1420 and 1430, an ion conductor 1440, a contact 1460, and anamorphous silicon diode 1470, such as a Schottky or p-n junction diode,formed between contact 1460 and electrode 1420. Rows and columns ofprogrammable structures 1400 may be fabricated into a high densityconfiguration to provide extremely large storage densities suitable formemory circuits. In general, the maximum storage density of memorydevices is limited by the size and complexity of the column and rowdecoder circuitry. However, a programmable structure storage stack canbe suitably fabricated overlying an integrated circuit with the entiresemiconductor chip area dedicated to row/column decode, senseamplifiers, and data management circuitry (not shown) since structure1400 need not use any substrate real estate. In this manner, storagedensities of many gigabits per square centimeter can be attained usingprogrammable structures of the present invention. Utilized in thismanner, the programmable structure is essentially an additive technologythat adds capability and functionality to existing semiconductorintegrated circuit technology.

FIG. 16 schematically illustrates a portion of a memory device includingstructure 1400 having an isolating p-n junction 1470 at an intersectionof a bit line 1510 and a word line 1520 of a memory circuit. FIG. 17illustrates an alternative isolation scheme employing a transistor 1610interposed between an electrode and a contact of a programmablestructure located at an intersection of a bit line 1610 and a word line1620 of a memory device.

FIG. 18 illustrates another array 1800, including programmablestructures 1802-1808 and Zener diodes (preferably with low reverse turnon voltage) 1810-1816, in accordance with the present invention. Aselected cell is written by, for example, making C₂=+V_(w)/2,R₂=−V_(w)/2 (where V_(w) is greater than or equal to the write thresholdof the device) with all other rows and columns held at zero volts sothat no other device in the array sees the full forward write voltage.To erase the selected cell, C₂ is set to −V_(E)/2, R₂=+V_(E)/2 (whereV_(E) is greater than or equal to the reverse turn-on voltage of theZener diode plus the erase threshold of the device) with all other rowsand columns again held at zero volts. The selected device is read usingmuch the same approach by making C₂=+V_(R)/2, R₂=−V_(R)/2 (where V_(R)is chosen to be below the write threshold) with all other rows andcolumns held at zero volts. It is also possible to use full rather thanhalf voltages when the non-selected rows and columns are allowed tofloat (via the use of tri-state drivers); e.g., C₂=+V_(w) and R₂=0 forwrite, C₂=0 and R₂=+V_(E) for erase, and C₂=+V_(R) and R₂=0 for read.

FIG. 19 illustrates another array 1900 that uses structures 1902-1908,which store information in the form of a change in capacitance ratherthan a change in resistance. This may be accomplished by the growth of anon-shorting electrodeposit, i.e., one that does not extend all the wayfrom anode to cathode. A non-shorting electrodeposit can be the resultof a reduced charge write pulse which produces a deposit that onlyextends a short distance from the cathode. A dielectric/tunnel barrierthat allows sufficient electron current to flow into the electrolyte butwhich prevents electrodeposition across the structure can also be usedto prevent a shorting electrodeposit from forming. Alternatively,electrodeposit growth across the electrolyte may be limited by providingan appropriate amount of oxidizable silver at the anode for region 160growth.

The capacitive storage structures 1902-1908 can be placed in an arraywithout diodes or other isolation devices as they will not createshorting paths from one row or column to another. Structures 1902-1908are programmed in the same way as the diode array described above, butthe erase voltage is now just the threshold voltage for dissolution ofthe electrodeposit.

FIGS. 20-23 illustrate, schematically, another technique for storinginformation in an array of lateral programmable structures that includean additional sensing electrode. Each structure includes an anode 2002,a cathode 2004, a sense electrode 2006, and a connector 2008.Information is stored as the capacitance of the geometric overlap of theelectrodeposit and sense electrode 2006, separated from each other by anappropriate dielectric. In this case, anode 2002 and cathode 2004 ineach device are fed by a row and column in the array. Since theelectrodes are coplanar in a lateral PMC device, one of the electrodes(e.g., the anode) must be connected to the array by way of a via contactor connector 2008 through the inter-metal dielectric. FIGS. 20-23 eachillustrate examples of individual device configuration schematics inwhich the anode is connected to a level 3 column (sense is level 1)(FIG. 20) or a level 1 column (sense is level 3) (FIG. 22).Configurations illustrated in FIGS. 21 and 23 have the sense line as alevel 1 or level 3 offset row (under or over the electrolyte) with theother electrodes configured as before. The cathode is a level 2 row inall cases (the dielectrics are not shown and the devices are not drawnto scale).

FIGS. 24 and 25 illustrate another array 2400 formed using theconfigurations presented in either FIG. 20 or FIG. 22. As illustrate inFIG. 25, a selected cell 2502 is written and erased via R₂ and C₂ asdescribed above, but is read by detecting a capacitance between R₂ andC₂*.

Another approach to forming a passive programmable array is to form anincomplete or otherwise high resistance electrodeposit to produce an“on” state with a resistance which is high enough to prevent sneakpaths. This may be achieved using a two terminal structure illustratedin FIG. 19 or three terminal structure illustrated in FIGS. 20-23.

FIG. 26 illustrates a two-terminal structure array 2600 in which apartial electrodeposit is grown the same way as discussed above, e.g.,C₂=+V_(w)/2, R₂=−V_(w)/2 with limited write time, limited oxidizablemetal, or with a resistive barrier between the electrodes (e.g., at thecathode). The resistance remains high after writing as theelectrodeposit does not bridge the electrodes. This relatively highresistance state can be differentiated from the full off state as thecurrent flow through the device will be altered by the presence of theelectrodeposit. One way to detect the presence of a partialelectrodeposit is to destructively read the device using an erase pulse,as described above Alternatively, a write operation can be similarlyused to destructively read the state.

FIG. 27 illustrates a three-terminal structure 2700 in accordance withyet another embodiment of the invention. Structure 2700 includes aresistive material such that a current path exists between theelectrodeposit and the sense electrode. In this case, a partialelectrodeposit is grown as described above, and information is stored asa change in resistance due to the geometric overlap of theelectrodeposit and the sense electrode. As illustrated in FIG. 27, aselected cell 2702 is written via R₂ and C₂ as before and the state(resistance) of the cell is sensed via R₂ and C₂*. The resistivity ofthe material between the electrodeposit and the sense electrode must behigh enough to prevent low resistance pathways forming between the senselines.

FIGS. 28 illustrates a lateral programmable structure 2800, including afirst electrode 2802, a second electrode 2804, an ion conductor 2806,and a conducting layer 2808, formed overlying a substrate 2810.

Ion conductor 2806 may be formed by, for example, photodissolving silverinto a Ch-rich Ge—Ch glass, where Ch is S or Se, until the resultingternary is “saturated” with the metal. In this case, the silversignificantly modifies the transport properties of the material and theavailability of mobile silver throughout the electrolyte is high. Whenan ion current flows, it is thought that the silver moves as acoordinated motion of ions—essentially a “ripple effect” analogous to ashift register in electronics. In order to be reduced, the ionic silvermust combine with the electron current from the cathode which means thatthe ions nearest the cathode will be reduced first and the conductingelectrodeposit so formed will become the source of cathodic electronsduring subsequent reduction. In this way, the growing electrodeposit“harvests” ions from the electrolyte as it grows out from the cathode,supplying electrons for the reduction of the local silver near itsboundaries and thereby further extending the electrodeposit across theelectrolyte. This process will continue until the source of oxidizablesilver is exhausted or until the resistance between the electrodes fallsto a point where the voltage drop is below the reduction/oxidationpotential. To join the growing electrodeposit, the individual ions onlyhave to move at most a few nm in the electrolyte. Those which move andare reduced are replaced in the electrolyte via the coordinated ionmotion from the anode so that the total quantity of metal in theelectrolyte is the original amount plus that added due to the ion flux.Accordingly, limiting factors for electrodeposition rate include ionmobility, ion displacement length, and supply of electrons to thereduction area. Because of the short distances involved in the iontransport and the considerable ion mobility at high field, the electroncurrent is likely to be the main factor affecting the rate ofelectrodeposition, particularly in long lateral devices. In suchdevices, since the electrolyte is typically formed as a thin film andhas a relatively high resistivity, the series resistance will tend to bevery large. For example., a 10 nm thick 100Ω·cm film will have a sheetresistance of about 10⁸Ω/square so that a 1V bias will initially onlyforce 10 nA in a square geometry device element which corresponds toabout 6.25×10¹⁰ electrons per second. If it is assumed that this is themaximum rate at which ions can be reduced in this case and that it staysconstant during electrodeposition, an electrodeposit with an effectivearea of about 10×10 μm² and a 1 nm thickness would take around 93 msecto form. If a desire is to produce a uniform electrodeposit over an areasuch as that described above, it will be unlikely to be realized due tothe elongated or dendritic growth that tends to occur—since the growingelectrodeposit is the main source of electrons once growth is initiated,all deposition takes place at its boundaries and particularly at itstip, further elongating the structure.

Accordingly, in accordance with one embodiment of the invention, toimprove both speed and electrodeposit coverage, an underlying conductor2808 is provided to distribute current and even the lateral electricfield as shown in FIG. 28.

In this example, the electrolyte and its coplanar anode and cathode areformed on a thin high resistance layer that separates the electrolytefrom a parallel conductor. The resistive layer parameters are chosen toprevent the electrodes from being shorted together by the conductinglayer but also to allow some current to flow to the conductor so that itcan be redistributed in the electrolyte. This additional source ofcurrent perpendicular to the plane of the electrolyte means that thecurrent is no longer limited by the lateral resistance of theelectrolyte and so the electrodeposition can proceed orders of magnitudefaster. Also, since the growing electrodeposit is no longer the onlysource of electrons, the electrodeposition effect may be much more twodimensional, allowing better coverage of the surface.

Note that this approach can also be used with vertical devices, such asstructure 2900, illustrated in FIG. 29. In this case the electrons willflow through the volume of ion conductor 2902 interposed between a firstelectrode 2904 and a second electrode 2906 via the parallel highconductivity regions illustrated by the arrows.

FIG. 30 illustrates a structure 3000, including electrodes 3002, 3004,ion conductor 3006, and a parallel conducting layer 3008. Electrode 3008is configured to facilitate a partial electrodeposit which only forms inor on part of the ion conductor. This may be achieved by having parallelconductor 3008 extending only part of the way from the cathode, asillustrated. If this electrode is connected to the cathode (or to aseparate supply of electrons), it will facilitate rapid and2-dimensional growth up to the point on the electrolyte where it ends.Beyond this point, the growth will be fed mostly by electrons from thesurface electrodeposit (now the cathode) and hence it will slow down.This way, we can grow only a partial 2-dimensional metal structure. Anon-bridging electrodeposit can also be attained in lateral devices orvertical devices with thick electrolyte by using the lowest possiblewrite voltage, i.e., a few mV above the reduction/oxidation potential,to keep the writing field low.

A programmable structure in accordance with the present invention may beused in many applications which would otherwise utilize traditionaltechnologies such as EEPROM, FLASH or DRAM. Advantages provided by thepresent invention over present memory techniques include, among otherthings, lower production cost and the ability to use flexiblefabrication techniques which are easily adaptable to a variety ofapplications. The programmable structures of the present invention areespecially advantageous in applications where cost is the primaryconcern, such as smart cards and electronic inventory tags. Also, anability to form the memory directly on a plastic card is a majoradvantage in these applications as this is generally not possible withother forms of semiconductor memories.

Further, in accordance with the programmable structures of the presentinvention, memory elements may be scaled to less than a few squaremicrons in size, the active portion of the device being less than onesquare micron. This provides a significant advantage over traditionalsemiconductor technologies in which each device and its associatedinterconnect can take up several tens of square microns.

Programmable structures and devices and system including theprogrammable structures described herein are advantageous because theprogrammable structures require relatively little internal voltage to.perform write and erase functions, require relatively little current toperform the write and erase functions, are relatively fast (both writeand read operations), require little to no refresh (even for “volatile”memory applications), can be formed in high-density arrays, arerelatively inexpensive to manufacture, are robust and shock resistant,and do not require a monocrystalline starting material and can thereforebe added to other electronic circuitry.

The programmable structure described herein can also be used tofabricate microstructures by dissolving the anode and plating outconductive material (e.g., silver) to redistribute mass of thestructure, wherein the amount of mass distributed depends on thefaradaic current applied across the electrodes. Such structures can beused to form non-volatile optical switches, MEMS relay contacts, and thelike. FIG. 31 illustrates an exemplary tunable resonator formed usingthe programmable structures of the present invention. Resonator 3100includes a mass reservoir or anode 3102, deposition electrode or cathode3104, a cantilever 3106, an input 3108, and an output 3110.

Although the present invention is set forth herein in the context of theappended drawing figures, it should be appreciated that the invention isnot limited to the specific form shown. For example, while theprogrammable structure is conveniently described above in connectionwith programmable memory devices, the invention is not so limited; thestructure of the present invention may additionally or alternatively beemployed as programmable active or passive devices within amicroelectronic circuit. Furthermore, although only some of the devicesare illustrated as including buffer, barrier, or transistor components,any of these components may be added to the devices of the presentinvention. Various other modifications, variations, and enhancements inthe design and arrangement of the method and apparatus set forth herein,may be made without departing from the spirit and scope of the presentinvention as set forth in the appended claims.

1. An array of rows and columns of programmable structures comprising: aprogrammable structure comprising a first electrode, a second electrode,and an ion conductor; and a Zener diode electrically coupled to theprogrammable structure.
 2. A microelectronic programmable structurecomprising: an ion conductor comprising an electrolyte and conductivematerial; an oxidizable electrode proximate the ion conductor; and anindifferent electrode proximate the ion conductor; wherein the ionconductor comprises a material selected from group consisting of GeO₂,As₂O₃, Ag₂O, Cu_((1,2))O, SiO₂, Ge_(x)S_(1-x), As_(x)S_(1-x),GexSe_(1-x), As_(x)Se_(1-x), Ge_(x)Te_(1-x), As_(x)Te_(1-x), and WO_(x).3. The array according to claim 1, wherein the ion conductor comprises amaterial selected from group consisting of GeO₂, As₂O₃, Ag₂O,Cu_((1,2))O, SiO₂, Ge_(x)S_(1-x), As_(x)S_(1-x), GexSe_(1-x),As_(x)Se_(1-x), Ge_(x)Te_(1-x), As_(x)Te_(1-x), and WO_(x).
 4. The arrayaccording to claim 1, wherein a first end of the programmable structureis in a column of programmable structures and a second end of theprogrammable structure is coupled to the Zener diode.
 5. The arrayaccording to claim 4, wherein a selected cell is written when the columnvoltage is at Vw/2, where Vw is greater than or equal to a writethreshold for the programmable structure.
 6. The array according toclaim 4, wherein a selected cell is written when the row voltage is at−Vw/2, where Vw is greater than or equal to a write threshold for theprogrammable structure.
 7. The array according to claim 4, wherein aselected cell is read when the column voltage is at Vr/2, where Vr isless than the write voltage of the programmable structure.
 8. The arrayaccording to claim 4, wherein a selected cell is read when the rowvoltage is at −Vr/2, where Vr is less than the write voltage of theprogrammable structure.
 9. The array according to claim 1, wherein aZener diode is electrically coupled to a row of programmable structures.10. The array according to claim 9, wherein a selected cell is erasedwhen the column voltage is at −Ve/2, where Ve is greater than or equalto a reverse turn-on voltage of the Zener diode plus the erase voltageof the programmable structure.
 11. The array according to claim 4,wherein a selected cell is erased when the row voltage is at Ve/2, whereVe is greater than or equal to a reverse turn-on voltage of the Zenerdiode plus the erase voltage of the programmable structure.
 12. Thearray according to claim 1, further comprising a contact, wherein theZener diode is interposed between the contact and one of the firstelectrode and the second electrode.
 13. An array comprising thestructure of claim
 2. 14. The microelectronic programmable structure ofclaim 2, further comprising a barrier layer interposed between one ofthe oxidizable electrode and the indifferent electrode and the ionconductor.
 15. The microelectronic programmable structure of claim 2,wherein the ion conductor comprises a doped oxide.
 16. Themicroelectronic programmable structure of claim 2, wherein the ionconductor comprises filler material.
 17. The microelectronicprogrammable structure of claim 2, wherein the ion conductor comprisesphase-separated material.
 18. The microelectronic programmable structureof claim 2, wherein at least a portion of the programmable structure isformed within a via of an insulating material.
 19. The microelectronicprogrammable structure of claim 2, further comprising a contact.